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ties, the work will never be dislodged. For a great rush
of air is generated in an aqueduct, strong enough to
break even stones, unless the water is softly and spar-
ingly let down from the head, and unless in elbows or
bending joints it be restrained by means of ligatures, or
a weight of ballast. In other respects it is similar to
one with leaden pipes. When the water is first let down
from the head, ashes are put in which will stop those
joints not sufficiently coated. Earthen pipes have these
advantages, first as to the work ; next, that if damaged
any one can repair it. Water conducted through earthen
pipes is more wholesome than that through lead ; indeed
that conveyed in lead must be injurious, because from it
white lead is obtained, and this is said to be injurious to
the human system. Hence, if what is generated from it
is pernicious, there can be no doubt that itself cannot be
a wholesome body. This may be verified by observing
the workers in lead, who are of a pallid colour; for in
casting lead, the fumes from it fixing on the different
members, and daily burning them, destroy the vigour of
the blood ; water should therefore on no account be con-
ducted in leaden pipes if we are desirous that it should
be wholesome. That the flavour of that conveyed in
earthen pipes is better, is shown at our daily meals, for
all those whose tables are furnished with silver vessels,
nevertheless use those made of earth, from the purity of
the flavour being preserved in them. If there be no
springs from which water can be obtained, it is necessary
to dig wells, on which every care is to be bestowed, and
the utmost ingenuity and discretion used in the examina¬
tion of the natural indications of the circumstances there-
about, inasmuch as the different sorts of soil which are
met with, are many and various. That, like every other
body, is composed of four elements; first of earth itself;
water, whence are the springs; heat, whence sulphur,
alum, and bitumen are generated ; and air, whence arise
great vapours, which, piercing through the pores to the
opening of wells, strike upon the excavators and suffocate